In textile terminology, the warp is the collection of lengthwise threads from which fabric is woven. That is to say, the warp is the thread which runs lengthwise of the loom. Typically, selected warp threads are raised by the heddles of the loom to allow the filling, or woof, to be carried between the alternate warp yarns. The alternate raising and lowering of the warps causes them to rub against one another, the rubbing causing abrasion of the threads. As they abrade, fine fibers are separated from the yarns, causing the yarns to cling together interfering with the passage of the shuttle. The warp yarns are also subjected to tensile and flexing forces which may cause them to break up during weaving.
This problem of abrasion of the warp in a loom has been dealt with by a process known as "slashing". When one slashes a warp, one applies to the warp threads a coating of size, which in effect glues loose fibrils to the body of the thread, and provides a degree of lubrication between the threads as they pass each other in the weaving operation. In early days of mechanized weaving, the sizing was applied to the warp on the loom by a man with a brush with a "slashing" motion. As modern high speed weaving was developed, the slashing step became a separate process. In the modern process, the warp is wound on a long mandrel called a "beam" in lengths of thousands of yards. Then the beam is unwound, the warp yarns being led through a machine called a "slasher". In the slasher, a solution of size material is applied to the threads. During slashing the yarns are passed over drying cylinders which remove the water and leave a coating of dry size on the yarn. The warp threads are separated one from another, and are rewound on another mandrel called the "loom beam". The loom beam itself is stored until a loom is ready to receive it for the weaving step.
In most cases, after a fabric has been woven, the warp size must be removed before the fabric is sent on to the finishing step, or to market. The step of removing the size is known as desizing. Typically, fabric from the loom is passed through a washing bath in which hot water dissolves the warp size. The cleaned fabric is dried and rewound. The wash water from the desizing bath, with its load of size washed out of the fabric, commonly must be treated before it is dumped into a nearby stream. Typical warp size is an organic material which, unless it is processed in a waste water treatment facility, will decompose in the stream, and rob the stream of dissolved oxygen, thereby creating a source of pollution. In technical language, the size creates "BOD" -- biological oxygen demand. The effluent from a large textile mill is therefore potentially a major source of stream pollution.
As a consequence of antipollution legislation, textile mills have been obliged to install expensive waste-treating plants, or else they have had to resort to special, and expensive, warp-sizing processes which are non-polluting to both air and water.
The substance most commonly used for a warp size is starch, which may be derived from any of several sources. The starch is first cooked, or gelatinized, which converts the starch into a viscous hydrophilic substance which will adhere to fibers of the warp threads, on which it is subsequently dried, and from which it can be readily removed, in the desizing step, in a hot water wash. The starch size may be modified by the addition of certain adjuncts as lubricants. The starch may be chemically modified to enhance certain of its properties. For example, the starch may be reacted with nitrogen-containing radicals to render the starch "soluble" in the sense that the starch can be readily dissolved in water and its gelatinous, viscous state, produced without previously cooking it. Although starch, and various modifications to it, makes a cheap and useful warp size, it cannot be reclaimed in the desizing step. The process of removing the starch from the woven fabric typically involves depolymerization of the starch with enzymes or other agents followed by washing away of the degradation products with water. The starch degradation products in the wash water are, of course, a total loss to the mill. Moreover, the wash water can no longer be discharged into a stream, as pointed out above, and therefore its disposal constitutes a major expense in the operation of the textile mill.
The disposal of desize wash water has become so great a problem that several radical alternatives to the conventional process have been proposed, and in some cases adopted. One solution to the problem is to dissolve a suitable size material in a nonflammable, non-aqueous solvent; for example, a chlorinated hydrocarbon. When warps slashed with such a solution are dried, the evaporated solvent must be recovered, for economy's sake and also to prevent air pollution. Then after weaving, fabric must be desized in the solvent, again requiring an expensive solvent recovery system. Finally, solvent must be reclaimed from the desize liquor by distillation, which step requires a large energy input.
Another approach to the problem involves using polyvinyl alcohol (PVA) dissolved in water. PVA in the desize wash liquor may be reclaimed for recycling. However, the reclamation step requires ultrafiltration, or reverse osmosis, an expensive and delicate process. Even though such reclamation plants cost millions of dollars, the disposal problem is so acute that several ultra-filtration plants are now in use.
The textile industry therefore needs a warp sizing process which will provide a suitable warp size, soluble in water, which can be recovered for recycling from the desizing wash liquor without the requirement of large amounts of energy, and without complex expensive process equipment.